The invention relates to novel electrochromic compounds, their use for elecrochromic devices and electrochromic devices containing these electrochromic compounds.
Electrochromic compounds are already known. See Topics in Current Chemistry, vol. 92, pp. 1-44, (1980), U.S. Pat. No. 4,902,108, WO 97/30134.
A common feature of most of these compounds is their symmetrical structure, for example, two identical rings are optionally linked to one another via a conjugated chain containing an even number of chain atoms or two identical heteroatoms are linked to one another via a conjugated chain or an aromatic or quinoid ring or form part of a heterocycle. This is illustrated by the examples below: 
In these formulae, R is a group such as an alkyl radical.
JP 60-223 886 describes the use of azines, for example of the formula 
in which R is a group such as alkyl, in an electrochromic display device. However, only one compound of this type is used as electrochromic compound in the device (corresponding to RED1) which involves the known disadvantages of high operating voltage and poor durability.
Electrochromic devices are already known. See D. Theis in Ullmann""s Encyclopaedia of Industrial Chemistry, vol. A 8, p. 622, Verlag Chemie 1987 and WO-A 94/23333. A distinction is made between two basic types:
Type 1: full-area electrochromic device.
Type 2: electrochromic display devices having structured electrodes.
Type 1 is used, for example, in electrically darkenable window panes or electrically dimmable automobile mirrors. Such devices are disclosed, for example, in U.S. Pat. No. 4,902,108.
Type 2 is used in segment and matrix displays. Such display devices are proposed, for example, in DE-A 196 31 728. Devices of this type can be observed transmissively or, in the case of a mirror coating, reflectively.
WO-A 94/23333 compares electrochromic materials having different constructions, but these are not used as display devices:
Construction a: the electrochromic substances are in the form of a fixed film or layer on the electrodes (Ullmann, see above).
Construction b: the electrochromic substances are deposited on the electrodes as a layer by the redox process (Ullmann, see above).
Construction c: the electrochromic substances remain permanently in solution.
For construction a), the best-known electrochromic material is the tungsten oxide/palladium hydride pair.
For construction b), viologens have been described as electrochromic substances. These devices are not self-erasing, i.e. the image produced remains after the current has been switched off and can only be erased again by reversing the voltage. Such devices are not particularly stable and do not allow a large number of switching cycles.
In addition, such cells constructed using tungsten oxide/palladium hydride in particular cannot be operated in transmitted light, but only reflectively, owing to light scattering at these electrochromic layers.
Elektrokhimiya 13, 404-408, U.S. Pat. No. 4,902,108 and U.S. Pat. No. 5,140,455 disclose an electrochromic system of the latter construction c). An electrochromic cell which is constructed from glass plates with a conductive coating includes a solution of a pair of electrochromic substances in an inert solvent.
The pair of electrochromic substances used is one electrochemically reversibly reducible substance and one reversibly oxidizable substance. Both substances are colorless or only weakly colored in the ground state. Under the action of an electric voltage, one substance is reduced and the other oxidized, both becoming colored. When the voltage is switched off, the ground state re-forms in the case of both substances, resulting in disappearance or fading of the color. 
U.S. Pat. No. 4,902,108 discloses that suitable pairs of redox substances are those in which the reducible substance has at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance correspondingly has at least two chemically reversible oxidation waves.
Various applications have been described for electrochromic cells of construction c). For example, they can be formed as automobile rear-view mirrors which can be darkened during night driving by application of a voltage and thus prevent dazzling by the headlamps of following vehicles (U.S. Pat. No. 3,280,701, U.S. Pat. No. 4,902,108 and EP-A 0 435 689). Furthermore, such cells can also be employed in window panes or automobile sunroofs, where they darken the sunlight after application of a voltage. Likewise described is the use of such devices as electrochromic display devices, for example in segment or matrix displays having structured electrodes (DE-A 196 31 728).
The electrochromic cells normally consist of a pair of glass plates, of which, in the case of the automobile mirror, one is mirrored. One side of these plates is coated over its surface with a light-transparent, electroconductive layer, for example indium-tin oxide (ITO), and in the case of display devices this conductive coating is divided into electrically separated segments provided with individual contacts. These plates are used to construct a cell by joining them by means of a sealing ring with their electroconductively coated sides facing one another to form a cell. This cell is filled with an electrochromic liquid via an opening, and the cell is tightly sealed. The two plates are connected to a voltage source via the ITO layers.
It has now been found that asymmetrical compounds composed of two different heterocyclic and/or carbocyclic rings linked via a conjugated chain containing an uneven number of chain atoms are likewise useful electrochromic compounds
The invention relates to an electrochromic device comprising (a) a pair of glass plates or a pair of plastic plates or a pair of plastic films, wherein (A) both plates or both films are transparent and at least one plate has a side with a transparent electrically conductive coating that is optionally divisible into separate, individually contacted area segments or (B) (i) one plate or one film is transparent, (ii) the other plate or film is mirrored and (iii) at least one plate or at least one film has a transparent, electrically conductive coating that is optionally divisible into separate, individually contacted area segments and, (b) a sealing ring for joining together the plates or the films on the sides having an conductive coating, wherein the two plates or films and the sealing ring form a volume, and (c) an electrochromic medium for filling the volume, wherein the electrochromic medium includes at least one oxidizable electrochromic substance RED1 and at least one reducible electrochromic substance OX2, wherein the substance RED1 is an electrochromic compound of the formula (CC): 
wherein
A is an aromatic or a quasi-aromatic, heterocyclic 5- or 6-membered ring in which X201 is located in the 2- or 4-position relative to the point of attachment,
B is an aromatic or a quasi-aromatic, carbocyclic or heterocyclic 5- or 6-membered ring in which Y201 is located in the 2- or 4-position relative to the point of attachment,
X201 is NR201, O or S,
Y201 is NR202R203, OR204 or SR205,
Z201 to Z203, independently of one another, are carbon or nitrogen,
R201 to R205, independently of one another, are alkyl, alkenyl, cycloalkyl, aralkyl or aryl, and
R202 to R205 may additionally be hydrogen,
i is an integer from 0 to 3, and
Z201 to Z203 may form a bridge with one another or with the rings A and/or B,
and/or the substance OX2 used is an electrochromic compound of the formula (CCl): 
in which
C is an aromatic or a quasi-aromatic, heterocyclic 5- or 6-membered ring in which X202 is located in the 2- or 4-position relative to the point of attachment,
D is an aromatic or a quasi-aromatic, carbocyclic or heterocyclic 5- or 6-membered ring in which Y202 is located in the 2- or 4-position relative to the point of attachment, or 
is xe2x95x90C(CN)2, xe2x95x90C(CN)COOalkyl or xe2x95x90C(COOalkyl)2,
X202 is N+R206, O+ or S+,
Y202 is NR207, O, S, Nxe2x80x94CN or C(CN)2,
Z204 to Z206, independently of one another, are carbon or nitrogen,
R206 and R207, independently of one another, are alkyl, alkenyl, cycloalkyl, aralkyl or aryl,
j is an integer from 0 to 3, and
Z204 to Z206 may form a bridge with one another or with the rings C and/or D,
where the rings A, B, C and D may carry further substituents.
The invention also relates to an electrochromic compound of the formula (CC).
The invention also relates to an electrochromic compound of the formula (L)
Yxe2x80x94[xe2x80x94(xe2x80x94Bxe2x80x94Zxe2x80x94)axe2x80x94(xe2x80x94Bxe2x80x94Yxe2x80x94)bxe2x80x94]cxe2x80x94Bxe2x80x94Zxe2x80x83xe2x80x83(L),
wherein Y and Z, independently of one another, are an OX2 or RED1 radical with the proviso that at least one Y is OX2 and at least one Z is RED1 or Y and Z are OX2,
wherein OX2 is the radical of a reversibly electrochemically reducible redox system, and
RED1 is the radical of a reversibly electrochemically oxidizable redox system,
B is a bridging unit,
c is an integer from 0 to 1000, and
a and b, independently of one another, are integers from 0 to 100,
wherein at least one OX2 corresponds to a radical of the formula (CCl) and/or at least one RED1 corresponds to a radical of the formula (CC).